The invention relates to a method of placing a ceramic coating on an article of manufacture including a substrate formed of a nickel or cobalt-based superalloy, the method which includes placing an anchoring layer on the substrate and placing the ceramic coating on the anchoring layer.
The invention in particular relates to an article of manufacture to be used as a gas turbine component which is subjected to a hot and oxidizing gas stream streaming along it in operation. Such gas turbine components include gas turbine airfoil components like blades and vanes as well as gas turbine heat shield components.
U.S. Pat. No. 4,055,705 to Stecura et al.; U.S. Pat. No. 4,321,310 to Ulion et al., and U.S. Pat. No. 4,321,311 to Strangman disclose coating systems for gas turbine components made from nickel or cobalt-based superalloys. A coating system described includes a thermal barrier layer made from ceramic, which in particular has a columnar grained structure, placed on a bonding layer or bond coating which in its turn is placed on the substrate and bonds the thermal barrier layer to the substrate. The bonding layer is made from an alloy of the MCrAlY type, namely an alloy containing chromium, aluminum and a rare earth metal such as yttrium in a base including at least one of iron, cobalt and nickel. Further elements can also be present in an MCrAlY alloy; examples are given below. An important feature of the bonding layer is a thin layer developed on the MCrAlY alloy and used for anchoring the thermal barrier layer. This layer may be alumina, alumina mixed with chromium oxide or a double layer of alumina facing the thermal barrier layer and chromium oxide facing the bonding layer, depending on the composition of the MCrAlY alloy and the temperature of the oxidizing environment where the layer is developed. Eventually, an alumina layer may be placed purposefully by a separate coating process like physical vapor deposition (PVD).
U.S. Pat. No. 5,238,752 to Duderstadt et al. discloses a coating system for a gas turbine component which also incorporates a ceramic thermal barrier layer and a bonding layer or bond coating bonding the thermal barrier layer to the substrate. The bonding layer is made from an intermetallic aluminide compound, in particular a nickel aluminide or a platinum aluminide. The bonding layer also has a thin alumina layer which serves to anchor the thermal barrier layer.
U.S. Pat. No. 5,262,245 to Ulion et al. describes a result of an effort to simplify coating systems incorporating thermal barrier layers for gas turbine components by avoiding a bonding layer to be placed below the thermal barrier layer. To this end, a composition for a superalloy is disclosed which may be used to form a substrate of a gas turbine component and which develops an alumina layer on its outer surfaces under a suitable treatment. That alumina layer is used to anchor a ceramic thermal barrier layer directly on the substrate, eliminating the need for a special bonding layer to be interposed between the substrate and the thermal barrier layer. In its broadest scope, the superalloy is formed essentially of, as specified in weight percent: 3 to 12 Cr, 3 to 10 W, 6 to 12 Ta, 4 to 7 Al, 0 to 15 Co, 0 to 3 Mo, 0 to 15 Re, 0 to 0.0020 B, 0 to 0.045 C, 0 to 0.8 Hf, 0 to 2 Nb, 0 to 1 V, 0 to 0.01 Zr, 0 to 0.07 Ti, 0 to 10 of the noble metals, 0 to 0.1 of the rare earth metals including Sc and Y, balance Ni.
U.S. Pat. No. 5,087,477 to Giggins, Jr., et al. shows a method for placing a ceramic thermal barrier layer on a gas turbine component by a physical vapor deposition process including evaporating compounds forming the thermal barrier layer with an electron beam and establishing an atmosphere having a controlled content of oxygen at the component to receive the thermal barrier layer.
U.S. Pat. No. 5,484,263 to B. A. Nagaraj et al. shows a metal article having a heat shield including: a barrier layer on a surface of the article and a reflective layer on the barrier layer. The reflective layer being formed from a material which is selected from the group formed of the noble metals, noble metal alloys and aluminum. The barrier layer may be an oxide or a nitride.
European Patent Application 0 446 988 A1 to V. Andoncecchi et al. shows a process for forming a silicon carbide coating on a nickel-based superalloy, including nitriding pretreatment of the superalloy or deposition of a film of titanium nitride on the superalloy by reactive sputtering. Thereafter a thin film of titanium nitride is being deposed using vapor-phase chemical deposition. After this the nickel-based superalloys annealed in a nitrogen and hydrogen atmosphere and a silicon carbide layer is placed using vapor-phase chemical deposition. With this process a coating is obtained wherein between a ceramic layer containing silicion carbide or silicion nitride and a superalloy an intermediate layer containing titanium nitride is being interposed.
European Patent Application 0 688 889 A1 to P. Broutin et al. shows a process for passivating the surface of a metallic article formed of a nickel-based superalloy. This metallic article is a stove-pipe or the like. On the substrate formed of the nickel-based superalloy a protective layer is applied containing silicion carbide or silicion nitride. Between the ceramic protective layer and the substrate an intermediate layer formed of aluminum nitride or titan aluminum nitride is interposed. The intermediate layer has a thickness of 0.15 to 5 xcexcm which is less than a thickness of the protective layer.
U.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712; and 5,401,307, all to Czech et al. disclose advanced coating systems for gas turbine components including protective coatings of MCrAlY alloys. The MCrAlY alloys disclosed have carefully balanced compositions to give exceptionally good resistance to corrosion and oxidation as well as an exceptionally good compatibility to the superalloys used for the substrates. The basis of the MCrAlY alloys is formed by nickel and/or cobalt. Additions of further elements, in particular silicon and rhenium, are also discussed. Rhenium in particular is shown to be a very advantageous additive. All MCrAlY alloys shown are also very suitable as bonding layers for anchoring thermal barrier layers, particularly in the context of the invention disclosed hereinbelow.
The aforementioned U.S. Pat. No. 5,401,307 also contains a survey over superalloys which are considered useful for forming gas turbine components that are subject to high mechanical and thermal loads during operation. Particularly, four classes of superalloys are given. The respective superalloys are formed essentially of, as specified in percent by weight:
1. 0.03 to 0.05 C, 18 to 19 Cr, 12 to 15 Co, 3 to 6 Mo, 1 to 1.5 W, 2 to 2.5 Al, 3 to 5 Ti, optional minor additions of Ta, Nb, B and/or Zr, balance Ni. These alloys are brought into shape by forging; examples are specified as Udimet 520 or Udimet 720 by usual standard.
2. 0.1 to 0.15 C, 18 to 22 Cr, 18 to 19 Co, 0 to 2 W, 0 to 4 Mo, 0 to 1.5 Ta, 0 to 1 Nb, 1 to 3 Al, 2 to 4 Ti, 0 to 0.75 Hf, optional minor additions of B and/or Zr, balance Ni. These alloys are cast into shape; examples are GTD 222, IN 939, IN 6203 DS and Udimet 500.
3. 0.07 to 0.1 C, 12 to 16 Cr, 8 to 10 Co, 1.5 to 2 Mo, 2.5 to 4 W, 1.5 to 5 Ta, 0 to 1 Nb, 3 to 4 Al, 3.5 to 5 Ti, 0 to 0.1 Zr, 0 to 1 Hf, an optional minor addition of B, balance Ni. These alloys are cast into shape; examples are IN 738 LC, GTD 111, IN 792 and PWA 1483 SX.
4. 0.2 to 0.7 C, 24 to 30 Cr, 10 to 11 Ni, 7 to 8 W, 0 to 4 Ta, 0 to 0.3 Al, 0 to 0.3 Ti, 0 to 0.6 Zr, an optional minor addition of B, balance cobalt. These alloys are cast into shape; examples are FSX 414, X 45, ECY 768 and MAR-M-509.
A standard practice in placing a thermal barrier coating on a substrate of an article of manufacture includes developing an oxide layer on the article, either by placing a suitable bonding layer on the article which develops the oxide layer on its surface under oxidizing conditions or by selecting a material for the article which is itself capable of developing an oxide layer on its surface. That oxide layer is then used to anchor the thermal barrier layer placed on it subsequently.
Under thermal load, diffusion processes will occur within the article. In particular, diffusion active chemical elements like hafnium, titanium, tungsten and silicon which form constituents of most superalloys used for the articles considered may penetrate the oxide layer and eventually migrate into the thermal barrier layer. The diffusion active chemical elements may cause damage to the thermal barrier layer by modifying and eventually worsening its essential properties. That applies in particular to a thermal barrier layer made from a zirconia compound like partly stabilized zirconia, since almost all zirconia compounds must rely on certain ingredients to define and stabilize their particular properties. The action of such ingredients is likely to be imparted by chemical elements migrating into a compound, be it by diffusion or otherwise. Likewise, the anchoring property of the oxide layer may be decreased partly or wholly by diffusion active chemical elements penetrating it.
In order to assure that a protective coating system including a thermal barrier layer placed on a substrate containing diffusion active chemical elements keeps its essential properties over a time period as long as may be desired, it is therefore material to prevent migration of diffusion active chemical elements.
Another relevant aspect in this context is the relatively poor thermal conductivity of alumina which can cause a hot zone to be created at the oxide layer in cooperation with heat reflection effects. Such a hot zone will cause high internal stresses to develop therewithin. These stresses may pertain considerably to a failure of a protective coating system including a thermal barrier layer on such an anchoring layer due to spallation which occurs within the anchoring layer or at an interface between the thermal barrier layer and the anchoring layer. In order to ensure a long life for the protective coating system and keep the oxidation of the bonding layer particularly low, care must be taken to transfer all the heat through the thermal barrier layer to the substrate and a cooling system which may be provided therein.
These aspects have, however, not yet received considerable attention by those working in the field. Heretofore, only an oxide layer has been given consideration to anchor a thermal barrier layer on a superalloy substrate regardless of its transmission of diffusing chemical elements to the thermal barrier layer and its poor thermal conductivity.
It is accordingly an object of the invention to provide a method of manufacturing an article with a protective coating system including an improved anchoring layer, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known products and methods of this general type and which keep to a minimum or prevent the transmission of diffusing elements through an anchoring layer to a thermal barrier layer and allow for sufficient heat transmission through the anchoring layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, an article of manufacture, including: a substrate formed of a nickel or cobalt-based superalloy; an anchoring layer placed on the substrate, the anchoring layer including a nitride compound; and a ceramic coating placed on the anchoring layer. Between the substrate and the anchoring layer there can be interposed a bonding layer.
A basic feature of the invention resides in replacing the oxide layer which has formed the anchoring layer within the protective coating system by an anchoring layer including a nitride compound, particularly aluminum nitride. Thereby, the relatively high thermal conductivity of aluminum nitride, which amounts up to 140 W/mK as opposed to a value between 30 W/mK at room temperature and 7.6 W/mK at 1000xc2x0 C. for alumina, as well as the relatively low ion transmission property of aluminum nitride are utilized to improve the relevant parameters of the anchoring layer. Particularly, the nitride compound is formed essentially of aluminum nitride.
The invention further relates to an article of manufacture, including a substrate formed of a nickel or cobalt-based superalloy, an anchoring layer disposed on the substrate, the anchoring layer including a nitride compound, and a ceramic coating disposed on the anchoring layer, whereby the nitride compound includes chromium nitride.
In accordance with an added embodiment of the invention, the anchoring layer is formed essentially of the nitride compound. In this context, it should be noted that aluminum in particular will preferably react with oxygen, if both nitrogen and oxygen are present. If oxygen and nitrogen are present in proportions similar to their proportions in air, it must be expected that only reactions between aluminum and oxygen will occur. This requires particular precautions to suppress the presence of oxygen if aluminum nitride is to be prepared by some reaction between elementary aluminum and nitrogen, particularly in the context of a reactive deposition process. Likewise, it must be expected that a compound formed by reacting nitrogen with aluminum contains a certain amount of compounds formed with oxygen, such as ordinary alumina. Such oxygen-containing compounds may eventually form inclusions within a matrix of aluminum nitride. In the present context, aluminum is a metal which has particular importance; however, the above consideration will apply to other metals as well, particularly to chromium.
In accordance with an additional embodiment of the invention, the article includes a diffusion active chemical element covered by the anchoring layer. The diffusion active chemical element is preferably an element selected from the group formed of hafnium, titanium, tungsten and silicon. In particular, the diffusion active element is contained in the substrate or a bonding layer disposed thereon.
Diffusion of the elements mentioned in the preceding paragraph is not considerably inhibited by ordinary alumina. Aluminum nitride, however, can act as an efficient diffusion barrier for these elements, since the nitrogen ions present within the aluminum nitride efficiently hinder a migration of atoms through the material. An additional advantage in this context is a reduced transmission of oxygen from the outside of the article and through the anchoring layer, since the nitrogen ions within the nitride compound also hinder the migration of oxygen ions. Thereby, it must be expected that oxidation of the material whereon the anchoring layer is disposed, namely a bonding layer or a substrate with special properties as explained, will occur at a rate which will be considerably lower than a rate of oxidation which must be expected with a usual anchoring layer in the form of oxides. In summary, both a depletion of a substrate or a bonding layer of diffusion active elements as well as oxidation of the substrate or bonding layer are inhibited, and the lifetime of the article with the protective coating system will be greatly enhanced.
In accordance with a further embodiment of the invention, the ceramic coating includes ZrO2. In a further development, the ceramic coating is formed essentially of ZrO2 and a stabilizer selected from the group formed of Y2O3, CeO2, LaO, CaO, Yb2O3 and MgO.
In a preferable embodiment, the anchoring layer has a thickness of less than 1 xcexcm. In particular, this thickness is between 0.1 gm and 0.4 Am. In any event, the thickness of the anchoring layer is selected by taking into account the relatively small coefficient of thermal extension of aluminum nitride which is 3.6xc3x9710xe2x88x926/K at room temperature to 5.6xc3x9710xe2x88x926/K at 1000xc2x0 C., to be compared with 6.2xc3x9710xe2x88x926/K at room temperature to 8.6xc3x9710xe2x88x926/K at 1000xc2x0 C. for alumina. In order to keep the mechanical stresses low in the anchoring layer, the thicknesses as mentioned are considered to be particularly effective.
In accordance with again a further embodiment of the invention, the article is provided with a bonding layer interposed between the substrate and the anchoring layer.
In preferred embodiments, the bonding layer is formed of a metal aluminide, or it is formed of an MCrAlY alloy.
In accordance with a particularly preferred embodiment of the invention, the ceramic coating has a columnar grained structure and the anchoring layer has a surface whereon the ceramic coating is placed, the surface having a surface roughness Ra less than 5 xcexcm. Preferably, the surface roughness Ra is less than 2 xcexcm. Particularly, the anchoring layer has a thickness more than 0.1 xcexcm. The parameter Ra characterizes a surface roughness in terms of an arithmetical mean deviation of the surface from a smooth mean profile along a measuring line of suitable length and form defined on the surface. Since Ra is thus an integral value, it is evident that it will be virtually independent of particular properties of the measuring line, provided that it is long enough to avoid influences of statistical fluctuations yet short enough to retain its significance for the surface under consideration.
The article as embodied according to the preceding paragraph features a ceramic coating which is of a columnar grained structure, which is expected to have superior mechanical properties. A columnar grained structure has crystallites in the form of small columns disposed one beside the other on the anchoring layer, thus allowing for almost free expansion of the substrate under thermal stress, assuring a particularly high lifetime for the protective coating system. Within that embodiment, bonding between the ceramic coating and the thermal barrier layer must be effected by a solid-state chemical bond. That bond is provided preferably by polishing the article within the course of placing (deposing, adhering) the different layers to achieve a surface roughness as specified.
In accordance with another preferred embodiment of the invention, the ceramic coating has an equiaxial structure and the anchoring layer has a surface whereon the ceramic coating is placed, the surface having a surface roughness Rz, greater than 35 xcexcm and a surface roughness Ra greater than 6 xcexcm, particularly a surface roughness Rz, between 50 xcexcm and 70 xcexcm and a surface roughness between Ra, between 9 xcexcm and 14 xcexcm. The parameter Ra has already been explained. The parameter Rz characterizes a surface roughness in terms of an average peak-to-valley height of the surface, where peak-to-valley heights of five individual measuring lines defined on the surface under consideration are averaged. Rz is thus a mean value for a maximum distance between a peak projecting out of the body having the surface and a valley projecting into the body. Both Ra and Rz are standard parameters, known in the art and defined as such in German norm DIN 4762, for example.
In the embodiment specified in the preceding paragraph, the ceramic coating has a particularly simple structure which allows for a particularly simple depositing process. As opposed to a ceramic coating with a columnar grained structure which must generally be applied by a special PVD process, a ceramic coating with an equiaxial structure can be placed by simple atmospheric plasma spraying. A ceramic coating of this type may not have the superior lifetime characteristic of a columnar grained ceramic coating, but it can be deposited in a particularly cheap way which makes it, within suitable compromises, also particularly useful. In this context, the anchoring layer, as well as the substrate itself or the bonding layer if present, can be left with a considerable surface roughness which may be obtained by simply applying the bonding layer by a process like vacuum plasma spraying and a-voiding any surface smoothing treatment.
The fairly rough surface of the anchoring layer will then retain the ceramic coating not only by a chemical bond, but also by mechanical clamping.
In accordance with yet an added embodiment of the invention the substrate, the bonding layer (if present), the anchoring layer and the ceramic coating form a gas turbine component. In particular, the gas turbine component is a gas turbine airfoil component including a mounting portion and an airfoil portion, the mounting portion being adapted to fixedly hold the component in operation and the airfoil portion being adapted to be exposed to a gas stream streaming along the component in operation, the anchoring layer and the ceramic layer placed on the airfoil portion.
With the above-mentioned and other objects in view, there is also provided, in accordance with the invention, a method of applying a ceramic coating to an article of manufacture having a substrate formed of a nickel or cobalt-based superalloy. Particularly, the substrate may have a bonding layer placed thereon, as described hereinabove. The method includes the following steps: placing (deposing) an anchoring layer including a nitride compound on a substrate formed of a nickel or cobalt-based superalloy; and placing a ceramic coating on the anchoring layer.
In accordance with an additional mode of the invention, the step of placing the anchoring layer is performed by physical vapor deposition. Preferably, a physical vapor deposition process including sputtering or electron beam evaporation is used.
In accordance with another mode of the invention, the step of placing the anchoring layer includes:
establishing an atmosphere containing nitrogen around the layer,
creating the anchoring layer by subjecting the layer and the atmosphere to an elevated temperature;
placing at least one metal to a surface of the substrate- and
reacting the metal with the nitrogen to form the nitride compound.
In accordance with a further mode of the invention, a plasma containing ionized nitrogen is formed around the substrate. Thereby reactions between nitrogen and metal compounds to form the desired nitride compound are facilitated.
In accordance with an additional mode of the invention, the metal is placed on the substrate by coating the substrate with the metal. Alternatively, the metal can be placed on the substrate by diffusing the metal out of the substrate or out of a bonding layer priorly placed on the substrate.
In accordance with yet another mode of the invention, the metal is selected from the group formed of aluminum and chromium.
In accordance with a particularly preferred mode of the invention, the surface is prepared on the substrate, eventually on a bonding layer placed on the substrate, the surface having a surface roughness Ra, less than 2 xcexcm, prior to placing the anchoring layer on the surface, and the ceramic layer is placed with a columnar grained structure. In this context, the surface is prepared preferably by polishing. Also preferably, a bonding layer is placed on the substrate, and the surface is prepared on the bonding layer. With further preference, the ceramic layer in this context is placed by physical vapor deposition, particularly to form a ceramic layer having a columnar grained structure. The formation of such structure may require that some kind of epitaxial growth is effected when placing the ceramic coating, to ensure that the desired columns of ceramic material are obtained.
In accordance with an alternative preferred mode of the invention, the surface is prepared on the substrate, the surface having a surface roughness Rz between 40 xcexcm and 50 xcexcm, prior to placing the anchoring layer on the surface, and the ceramic layer is placed with an equiaxial structure. Particularly, the surface is prepared by placing a bonding layer on the substrate by vacuum plasma spraying, establishing the surface on the bonding layer and leaving the surface without smoothing treatment. In this context, the ceramic layer may be placed by atmospheric plasma spraying to obtain an equiaxial structure.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of manufacturing an article with a protective coating system including an improved anchoring layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.